What financial secrets is your partner keeping from you?

Nothing says “what’s mine is yours” better than a joint bank account, especially if one of you is earning a lot more than the other.

It seems, however, that sharing lives, and beds, is one thing, but we are not quite so open with our finances.

Research into couples’ financial habits shows that almost one-third of partners have savings they keep secret, with some hiding as much as £50,000.

And it is not just cash. The study of 1,000 people commissioned by Prudential found that couples had all sorts of financial secrets.

More than a fifth said their partners did not know their salaries, while a similar number hid credit card debts and personal loans.

It seems that today’s couples behave differently from those of the 1970s and 1980s. Back then, joint accounts were the norm as women started contributing to the household income. Now many of us insist on keeping our financial independence.

Having separate accounts is not in itself a bad thing (I have one) as it allows both partners some autonomy. Hiding money from your partner is very different, though.

The Prudential found that men were likely to squirrel away for a rainy day while women who saved in secret did so in case of a break-up — a so-called “running away fund”.

Psychologists suggest that having only separate accounts means you are unsure about the relationship; this view holds that a joint account is the first stage in weaving your lives together.

Good reasons for having separate accounts, to my mind, include control — the fact that some people don’t feel their partners would make the right financial decisions. This argument can be strengthened when one person has a different tolerance to risk from the other.

Greg Davies, head of behavioural science at Oxford Risk, has studied the financial personalities of thousands of retail investors and notes that people typically fall at different points on what he calls the “spendthrift/tightwad scale”.

While it might be prudent for one person to take charge of the family finances, this is a decision that should be taken by a couple together. What is more, the finances should be transparent.

The problem is that many of us are uncomfortable talking about money, even with the person to whom we’re closest. Almost one in 12 Britons think couples should wait until they have been in a relationship for up to five years before discussing finances, says research by short-term loan provider Wonga.

Other complications ensue where people, like me, have grown up in families in which only one parent worked, since they are accustomed to see this as the norm in family life. Now it is more likely that both partners earn, particularly before children come into the picture.

There are seldom rules about how to split money, even within a relationship. I’ve been married for almost 10 years but I’m always inclined to split the restaurant bill 50:50. My husband is less rigid, preferring for one of us, it doesn’t matter who, to pay — but this changes every time we go out.

I believe it’s important for women to retain financial independence if they get married and have children. When one person is at home looking after the children while the other is bringing in a salary then the often unspoken norm is that the one who is working controls the money. That can strain a relationship, however.

Friends who have become full-time mothers after working have struggled silently with the lack of personal money. They feel they are losing their sense of self whenever they ask their partner for cash and can resent having to justify their spending.

For most, having a joint account for major bills into which both pay most of their money — with separate accounts that allow individual “pocket money” — appears to work best.

You will, of course, need to work out how you buy each other treats: is that luxury evening out a present from one to the other or a joint expense? Perhaps that is one way to find whether you need the luxury handbag or new golf clubs.

Experts say a clear, open, honest conversation about money is essential but how does that work in practice? Does one partner need permission to spend? Does another get resentful if their partner puts in less money?

I would not care for my husband to question my decisions on what I buy for our children or the household. And I’m glad I have a separate account for clothes and other personal purchases.

Being a personal finance journalist, I’m aware of money but I’m also relaxed about keeping some things to myself. I’m sure my husband spends more on a night out with friends than I would but I don’t see the need to interrogate him for the details.

Those looking for the cause of a doomed relationship are unlikely to find it in separate bank accounts. If your finances are healthy and you’re open with each other, there is nothing wrong with financial demarcation.

Lucy Warwick-Ching is FT Money’s digital and communities editor; lucy.warwick-ching@ft.com; Twitter @WarwickChing